1. Field of the Invention
The present invention relates to high energy laser systems, to beam delivery systems, and to laser peening systems suitable for use with stationary targets.
2. Description of Related Art
The use of mechanical shocks to form metals and to improve their surface properties has been realized for ages. In current industrial practice, a peening treatment of metal surfaces is accomplished by using high velocity shot. Treatment improves surface properties and, very importantly for many applications, results in a part displaying significantly improved resistance to fatigue and corrosion failure. A wide range of workpieces are shot peened in the aerospace and automotive industries. However, for many applications, shot peening does not provide sufficiently intense or deep treatment, or cannot be used because of its detrimental effect on the surface finish.
With the invention of the laser, it was rapidly recognized that the intense shocks required for peening, or other processing, could be achieved by means of a laser-driven, tamped plasma. B. P. Fairand, et al., “Laser Shot Induced Microstructural and Mechanical Property Changes in 7075 Aluminum,” Journal of Applied Physics, Vol. 43, No. 9, p. 3893, September 1972. Typically, a plasma shock of 10 kB to 30 kB is generated at metal surfaces by means of high energy density (about 200 j/cm2), short pulse length (about 30 nanoseconds) lasers. A thin layer of metal tape, black paint or other absorbing material on the metal surface provides an absorber to prevent ablation of the metal. A confining or tamping material such as water covers the surface layer providing an increased intensity shock. During the process of laser peening, an intense pressure pulse propagates into the part. If the internal pressure of the metal subjected to this pulse exceeds its elastic limit, plastic deformation occurs. Surrounding material not impacted by the pressure wave resists the resulting deformation, leaving a residual compressive stress in the treated volume. In testing, this treatment has been shown to be superior for strengthening workpieces from fatigue and corrosion failure. Laser peening is also used for forming and texturing surfaces.
One laser system which has been utilized for this purpose is described in our prior U.S. Pat. No. 5,239,408, entitled HIGH POWER, HIGH BEAM QUALITY REGENERATIVE AMPLIFIER. The laser system described in the just cited '408 patent comprises a high power amplifier in a master oscillator/power amplifier MOPA configuration capable of producing output pulses greater than 20 Joules per pulse with the pulse width on the order of 10 to 30 nanoseconds or less using a wavefront correcting configuration based on a stimulated Brillouin scattering SBS phase conjugator/mirror system.
There are several aspects of the laser peening acoustic wave that impact the effectiveness of the process and the depth and magnitude of the residual compressive stress. In the simplest description, as above, a pressure pulse propagates into the material. However, there are also traveling tensile waves that affect the process. At the end of the laser-induced pressure pulse, the elastic component of the material displacement causes the surface to rebound. This launches a tensile pulse (sometimes referred to as a rarefaction wave) into the metal, propagating just behind the compression pulse. Since the tensile pulse is traveling through higher density material, just previously compressed by the pressure pulse, it propagates faster and can eventually overtake the pressure pulse and thereby reduce its magnitude. The combination of this phenomenon with the natural spread (diffraction) of the acoustic wave, limits the depth of plastic deformation that can be achieved in a thick (approximately >5 mm) sample.
In a thin section (approximately <5 mm), the interaction of the acoustic waves inside the part becomes more complex. When the pressure pulse reaches the opposite side of the component, it causes an outward displacement of the unconstrained back surface, resulting in a reflected tensile pulse that propagates back into the metal. In a way analogous to the front surface, the back surface then rebounds, generating a counter-propagating pressure pulse that follows the reflected tensile pulse. Another description of this process is that of the simple reflection of the acoustic wave from the back surface of the part. Since the speed of sound is higher in the metal than in air, the resulting impedance mismatch causes the reflected wave to be inverted, as just described.
The reflected tensile wave has a number of undesirable effects. First, it can interact with the forward-going compression wave adjacent to the reflection boundary, reducing its magnitude and limiting the ability to generate through-thickness residual compressive stress. Of more concern, however, is the possibility that the reflected tensile pulse combines with the forward propagating tensile pulse (described above) in the interior of the part. In a process called spalling, the sum of these two tensile waves can exceed the yield strength of the material and cause internal cracks, distributed through the thickness.
The reflection of acoustic waves from the interior surfaces of a component can be suppressed by placing a block of similar material (same speed of sound) in intimate contact with the surface of concern. See, U.S. Pat. No. 4,401,477 by Clauer et al. This block of material is often referred to as a momentum trap. In one approach, the trap material is a relatively thin spring-loaded disk, the displacement of which is intended to carry off the momentum of the impinging wave. In a second approach, the trap is sufficiently large to allow the acoustic waves coupled out from the treated part to dissipate before encountering another material boundary. However, reentrant geometries or closely spaced components (such as fixed jet engine compressor blades) may provide space limitations that rule out large momentum traps.
The challenge in both cases is to achieve a very accurate shape to provide intimate contact to parts with complex surface geometries and to maintain this contact on a consistent basis during processing. Placing a liquid metal (e.g. mercury) or a liquid slurry of metal particles in a thin film between the momentum dump and the processed part has been proposed. See, U.S. Pat. No. 6,805,970 by Hackel et al.
Another approach to the design of a momentum trap uses flowing liquids. Flowing liquids clearly have the advantage of being able to readily conform to complex shapes. However, the difference in sound speed between the metal and the liquid does not provide an optimal impedance match and some reflection still takes place. Furthermore, the thickness of the flowing liquid stream is limited by the viscosity and surface tension characteristics of the liquid, typically to no more than a few millimeters. The acoustic absorption losses in the liquid are small and therefore reflections from the liquid/air interface on the backside of the water stream also remain a concern. The Hackel et al. patent cited above proposes replacing water as a momentum trap liquid with Fluorinert™, a chemically stable fluorocarbon with a density of almost twice that of water. However, this is still only half the density of a typical titanium alloy and the issues of impedance matching and layer thickness remain.
The direct use of liquid metals, such as mercury, either flowing in a stream or held in a reservoir against the back of the part, has been suggested. See, U.S. Pat. No. 6,559,415 by Mannava et al. In the same patent, the direct use of lubricants containing a slurry of metal powder is also proposed. These are clearly not practical approaches due to issues of process cell contamination by the airborne dispersal of the liquid material during the peening process, and due to the difficulty of recovery and reuse of these fluids.
It is desirable therefore to provide a momentum trap design suitable for use in manufacturing devices including metallic bodies that are laser peened.